Having premade entities created during the Linq process in .Net really comes in handy. But did you know that one can extend those entities further outside of of the generated dbml file? By using the concept introduced in C# 2.0 of a partial class, where one can have business logic methods and other properties which are not generated but are none the less relevant in an Object Oriented fashion it can be achieved.

As mentioned the entities are generated and nothing is worse than having the generator throw away user code placed in the generated file after a regeneration. To allow for generated and user code to coexist hand in hand was accomplished by partial classes. That is where the partial keyword is appended to a class which allows for the class to have a subset of functionality in a different code file. The generated code file resides in one file and the user code lives elsewhere in a different file allowing them co-exist.

I have used this process to put validation logic on an entity. That logic was used to verify that the differing properties which the user had entered values against matched business rules. Those rules could not be entirely expressed in a database schema. It was very Handy!

Example

A user posted a question on the MSDN Forums as to how one could achieve indexing into a Linq generated class entity where each property represented a numerated indexed field. The below code shows the use of a partial class in a separate file which the user could have an index-able entity.